ASPIRE is a quarterly magazine published by PCI in cooperation with the associations of the National Concrete Bridge Council. The editorial content focuses on the latest technology and key issues in the Concrete Bridge Industry.
Issue link: http://www.aspiremagazinebyengineers.com/i/768999
CONCRETE BRIDGE TECHNOLOGY The Quality Partnership: Ensuring the Performance and Reliability of Post-Tensioning Installation and Grouting by Theodore L. Neff, Post-Tensioning Institute Concerns are being raised about posttensioning (PT) grouting and installation, even though the majority of PT bridges worldwide are performing well and without problems. Why then the concern? Several highly publicized problems have focused attention on PT bridge construction, and particularly on grouting. While actual cases of poor grouting are very few in number, these problems have shown that quality has not always been achieved and better reliability is needed. Quality and Reliability In an ideal world, everyone on the project team would share responsibility and ensure that everything is done right and with the highest quality possible. But as experience has shown in all aspects of construction, “stuff” happens. Whether it be from inattention, ignorance, inexperience, neglect, competitive pressures or simply bad luck, things do not always go according to plan. Identifying Problems and Ensuring Quality Construction Recognizing that some problems are inevitable, it is critical that appropriate controls are in place to ensure that these situations are identified and appropriate corrective/preventive actions are taken in a timely manner. To do so effectively requires the following: • Good plans, specifications, and project details • Quality control by the contractor and all of its subcontractors and suppliers • Quality assurance by the owner or specifier • Education and tra ining of all personnel involved in the construction process All are needed to consistently achieve a high degree of reliability. Plans, Specifications, and Details Plans, specifications, and details are typically the responsibility of the owner’s engineer. Suitable plans and details greatly impact constructability and are the foundation for achieving quality. Poor details, such as improper location of grout inlets and outlets, may lead to voids and difficulty in completely filling a duct with grout. Furthermore, effective specifications “set the bar” for quality and ensure that the owner’s performance and reliability objectives are met. Alternatively, lax specifications may put contractors, who are trying to do a good job, at a competitive disadvantage, and entice them to a hurried schedule without properly controlled processes to stay competitive. Contractor Quality Control The contractor must have adequate quality control (QC) measures in place to ensure that the completed construction meets the owner’s requirements as detailed in the plans, specifications, and other contract documents. This responsibility also applies to all of the suppliers and subcontractors on the project. Often materials and products are pre-approved through ASTM testing or a qualification program before the project begins. However, as was learned the hard way with prepackaged grout in the PT industry, it is important that manufacturers have an ongoing QC program to ensure that their materials and products continue to meet project requirements throughout production and not just during initial qualification. Contractor QC must take into account the coordination and interaction of different subcontractors and operations that impact quality of the post-tensioning construction. If left solely to a subcontractor, there are often factors that are beyond the subcontractor’s control. For example, reinforcement conflicts or misaligned ducts in precast concrete elements can lead to difficulties in PT installation and subsequent grouting. In these cases, the PT installation subcontractor is responsible for installation and grouting but may not have any control over the quality of other subcontractors’ operations. The contractor’s oversight and jobsite testing (for example, flow cone, density, and pressure testing) at critical points is essential to ensuring the quality of subcontractor work. Owner’s Quality Assurance Quality assurance (QA) is normally the responsibility of the owner or its representative. Acceptance testing and related payment items are keys to a successful QA program. Enforcement of specification requirements is vital to achieving quality on the immediate job, as well as future projects.